1. Field of the Invention
Various types of modified starches have been used in treating paper in order to improve its strength and surface characteristics. Modified starch additives are commonly introduced into the "wet end" of the manufacturing process and they are also commonly coated onto formed cellulosic sheets in size presses and other types of coating apparatus. Since at least the 1960's, the utilization of cationic starches as wet end additives has become popular due to certain important advantages which they exhibit in comparison with both unmodified starches and other types of modified starches.
In these times of increasing awareness of the environmental aspects of manufacturing processes, it has been noted that cationic starches reduce BOD pollution problems. Cationic starches help retain fiber fines. They exhibit superior absorbancy onto anionic cellulosic fibers and onto common pigments. This improved absorbance and attendant superior pigment binding efficiency means that lesser amounts of cationic starch can be used to meet sheet specifications. Since lesser amounts of starch are used and a greater percentage of the starch used is bound onto the cellulose fiber, lesser amounts of BOD-producing starch will be present in effluents produced during paper manufacture.
These interesting and important properties of cationic starches are believed to be due to attraction between the starch and both anionically charged pigment and fiber resulting in improved chemical anchoring of coatings to the cellulose. Of course, the improved anchoring exhibited by cationic starches would be desirable at the coating stage as well as at the wet end. Indeed, it has been found that when cationic starch is used as a surface size, more starch is retained on or near the surface of the sheet than is retained with conventional starches. Thus, less starch is needed to maintain surface strength and quality. Since the cationic starch is tightly bound to the paper fiber, it is not removed during the repulping of broke. Hence, starch from the broke is recovered in the papermaking operation rather than being lost in the mill effluent. This attachment to the fiber coupled with the fact that reduced amounts of cationic starch are needed at the size press results in significant reductions in mill effluent BOD.
In addition, cationic surface sizing results in improved printing properties. These improved properties are believed to be due to a combination of fiber bonding and surface orientation resulting in more uniform starch concentration on the paper surface, and hence more uniform ink receptivity and improved ink hold-out. The improved fiber bonding provides a strong paper surface. Among the various improved printing characteristics experienced with the use of cationic starches are: better printing uniformity, better print definition, greater depth of color, reduced ink show-through, fewer pick-outs on offset press and reduced dusting.
In spite of their many advantages, in practice cationic starches have been found to have a serious drawback when used in coating or sizing. Cationic starches are generally manufactured by an approximately 12 hour process of swelling starch particles in a slurry using an alkaline agent to raise the pH, reacting the starch with tertiary or quaternary amines, neutralizing, washing and filtering. Although the presence of small amounts of the alkaline agent contributes to the swelling of the starch particles, the bulk of the alkaline agent is present in order to render the non-ionic end groups of the starch anionic thereby facilitating the addition of the cationic material which gives the starch its cationic character. As discussed below, considerable amounts of alkaline agent are required in order to accomplish the latter objective. The use of large amounts of acid to neutralize the alkaline agent is expensive and impractical and tends to introduce significant amounts of unwanted salts. The presence of unneutralized alkaline agent, on the other hand, significantly discolors the starch thereby impairing the color and brightness of the starch-coated sheet.
Since the present invention has eliminated the need for the alkaline agent to facilitate the starch acceptance of cationic material, both the neutralization and the significant discoloration problems have been eliminated. Small amounts of alkaline agent may optionally be employed in order to improve starch swelling without impairing the coloration of the sheet or the cationic addition to the starch. The present invention comprises an important addition to the art-- it discloses a new and useful paper coating agent which eliminates problems generally inherent in cationic starch coating agents.
2. Description of the Prior Art
Various cationic starches have heretofore been described. Among the patented prior art, the following patents are of possible interest:
U.S. Pat. No. 2,813,093-- Caldwell et al., (National Starch)-- 1957
Granular starch plus a tertiary amine hydrochloride (e.g., b-diethyl aminoethyl chloride).
U.S. Pat. No. 2,876,217-- Paschall (Corn Products)-- 1959
Granular starch ethers produced by reaction of a tertiary amine (e.g., trimethyl amine) with epichlorohydrin (epi) and suppressing cross-linking by vacuum or solvent elimination of excess epi.
U.S. Pat. No. 2,995,513-- Paschall et al., (Corn Products)-- 1961
Gelatinized starch ethers produced by epichlorohydrin plus a tertiary amine or a quaternary amine salt (e.g., trimethyl amine hydrochloride). The degree of substitution was D.S.= 0.3 to 0.5.
Canada Pat. No. 715,586-- Paschall et al., (Corn Products)-- 1965
Granular or ungelatinized starch ether prepared from an epi-tertiary amine condensate where the tertiary amine contains two or three methyl groups and is condensed equimolar with epichlorohydrin; also, there is a provision to remove unreacted epi by vacuum or solvent extraction, thus preventing cross-linking by unreacted epi.
Australia Pat. No. 404,812-- Brown (Australian Paper Manufacturers Limited)-- 1969
Gelatinized starch reacted in situ with an epichlorohydrin-ammonia condensation product.
U.S. Pat. No. 3,666,751-- Jarowenko et al., (National Starch)-- 1972
Liquid starch products similar to Brown above prepared by reacting starch with epichlorohydrin-ammonium hydroxide condensates.
U.S. Pat. No. 3,674,725-- Aitken et al., (Nalco)-- 1972
Method of cationization of starch which comprises reacting starch under alkaline conditions with a polymer formed from polyepichlorohydrin and an amine (e.g., trimethylamine, dimethylamine, etc.).
U.S. Pat. No. 3,738,945-- Panzer et al.--
Utilizes reaction products of epichlorohydrin and secondary amines.
U.S. Pat. No. 3,884,909-- Kightlinger et al., (Standard Brands)-- 1975
A cationic starch suitable for paper sizing produced by (1) reacting starch with an alkali - catalyzable cross-linking agent and the reaction product of the cross-linking agent with ammonia or with an amine and ( 2) depolymerizing the resulting cationic cross-linked product.
U.S. Pat. No. 3,854,970-- Aitken (Nalco)-- 1974
A cationic starch useful as a wet end additive produced by treating starch with an alkaline agent and a condensate of generally equimolar quantities of epichlorohydrin and dimethylamine wherein up to 30% molar ammonia may be substituted for a like amount of the dimethylamine. Usefulness as a wet end pulp additive is disclosed in the dosage range of 3-50 pounds per ton.
The present invention differs from most of the prior art cited in that it is designed for a liquid cationic starch treated either with a binary condensate of epichlorohydrin (EPI) and dimethylamine (DMA) or for a modified ternary condensate wherein up to 30% molar ammonia is substituted for a like molar amount of DMA. In contrast, 3,666,751 above is a binary condensate of EPI and ammonium hydroxide and the closest teaching to this invention is believed to be Column 4, lines 35-37, which indicates a possible inclusion of up to 15% of a cross-linking inhibitor specifying ethylamine and dimethylamine. 3,674,725 above produces a cationic starch additive from related starting materials but used polyepichlorohydrin instead of the present EPI.
The present invention also differs significantly from 3,854,970. It will be noted that 3,854,970 requires the use of a strong alkaline agent as discussed in Column 3, lines 17-23. It is explained there that the minimum pH must be at least 11, and that, when using a preferred alkali metal hydroxide such as NaOH, 5-8% by weight is applied. It is further explained in the 3,854,970 patent that 3-50 pounds per ton of the cationic starch are useful in wet end treatment (Column 3, lines 53-54). This corresponds to an NaOH level of from 0.0075-2.5%. Such levels of alkaline agent do not adversely affect paper coloration. However, the quantities of cationic starch required at the size press, which are far greater than the quantities required for wet end treatment, significantly discolor the paper product and otherwise adversely affect its properties.
Hence, the present invention entails a method of paper sizing utilizing a cationic starch similar to that disclosed in 3,854,970. The present invention, however, differs significantly from 3,854,970 in that the requirement for an alkaline agent has been eliminated. Thus, the present invention makes available to the art an important cationic starch which heretofore was unavailable due to the adverse affects of high amounts of alkaline agent which were heretofore necessary to produce cationic starch.